1. Field of Invention
The invention pertains to a process for determining the acidity of a washing solution for removing adhering acid or base from a fibre.
2. Description of Related Art
There are different ways to produce fibres. The most widely used fibre-forming processes are melt spinning and solution spinning. In both processes, the polymer is extruded through the orifices of a spinneret. In melt spinning molten polymer is used, while in solution spinning use is made of a spinning dope solution comprising a dispersed or dissolved polymer. After the extrusion of the molten polymer or the spinning dope solution, different spinning techniques can be applied to produce the fibres.
Spinning dope solutions can be acidic. For example, a polymer comprising poly(p-phenylene terephthalamide) is usually dispersed in a sulfuric acid solution for the production of aramid fibres. After the solution has been extruded and a spinning technique applied, traces of the acidic solution often adhere to the produced fibres. These traces need to be removed in order to prevent hydrolytic degradation of the fibres, which occurs especially above 20xc2x0 C.
Commonly, a washing solution is used to remove adhering acids or bases from a fibre. It is important to measure and adjust the acidity of the washing solution in such a way that fibres with a minimum amount of adhering acid or base can be produced. The acidity can be measured directly by using a titration method or a pH measurement. However, pH measurements are not preferred for accurate determination of the acidity because there is a necessity to calibrate the pH electrode very often and because the inaccuracy of the measurement increases as the pH-value deviates further from pH 6 to 8. Most commonly, the acidity of the washing solution is measured using a titration method, as described in M. Valcarcel et al., Techniques and Instrumentation in Analytical Chemistry, Vol. 9, Elsevier, Amsterdam, 1988. However, the information time of said titration method is relatively long. Therefore, it is very difficult or even impossible to adjust the acidity quickly enough to obtain fibres with a minimum amount of adhering acid or base. Another disadvantage of said method is the need to use reagents when carrying out a titration. Furthermore, a titration device used for measuring the acidity of a washing solution on-line requires relatively frequent maintenance.
The acidity of a washing solution can also be determined indirectly by using another analytical method such as a density, conductivity, ultrasonic sound or refractive index measurement. However, the results obtained carrying out one of these measurements in a washing solution are insufficient for accurate determination of the acidity when there are variable concentrations of salts or compounds present in the solution. For example, if sulfuric acid is used for the preparation of a spinning dope solution for the production of fibres, and traces of the solution still adhere to the produced fibres before washing, then variable concentrations of sodium sulfate will often be present in a washing solution for these fibres after interaction of the sulfuric acid with, for example, caustic soda.
It is accordingly the principal object of the present invention to provide a method for rapid and accurate determination of the acidity of a washing solution for the production of fibres with a minimum amount of adhering acid or base. Preferably, the method is applicable on-line. Furthermore, the device used for the determination preferably requires less maintenance than do the commercially available devices presently employed in the commonly used titration methods.
Surprisingly, a process has been found that permits rapid and accurate determination of the acidity of a washing solution for removing adhering acid or base from a fibre. The process is characterized in that the determination is carried out using a combination of at least two different analytical methods, at least one of which is selected from the density, conductivity, ultrasonic sound, and refractive index measurements. Rapid and accurate determination makes it possible to adjust the acidity of a washing solution in such a way that fibres with a minimum amount of adhering acid or base can be produced.
By fibres with a minimum amount of adhering acid or base is meant that the molar ratio of the OHxe2x88x92 ions to the H+ ions present in the medium adhering to said fibres is about 1, with the provisio that these ions are not chemically bound to the fibres. This ratio can be determined indirectly by analyzing the amount of counter-ions of the OHxe2x88x92 and the H+ ions in the medium, for example the amount of Na+ ions when NaOH is used for washing the fibre and the amount of SO42xe2x88x92 ions when the fibre is spun from a sulfuric acid solution. In this specific example a Na/S ratio can be determined by using X-ray diffraction spectroscopy (XRF). However, any other suitable analytical method may also be used.
Preferably, the combination according to the invention comprises at least two analytical methods selected from the density, conductivity, ultrasonic sound and refractive index measurements. The analytical devices used for said measurements require less maintenance and are easier to calibrate than the commercially available devices presently employed in the commonly used titration methods. Furthermore, the devices used for these measurements are most practical when it comes to installation for measuring on-line.
Even more preferably, the combination according to the invention comprises a conductivity measurement or a density measurement, because such a combination enables highly accurate determination of the acidity of a washing solution for fibres. This is especially the case for a process according to the invention wherein the combination comprises at least : 1) a conductivity measurement and an ultrasonic sound measurement, 2) a conductivity measurement and a density measurement, or 3) a density measurement and an ultrasonic sound measurement. Most preferably, the combination comprises at least a conductivity measurement and an ultrasonic sound measurement, because the devices used for these measurements are easiest to clean and require the least maintenance. This is especially desirable when the measurements are carried out on-line.
The measurements xe2x80x9care carried out on-linexe2x80x9d means that they are carried out continuously or at certain time intervals in a washing solution or any process stream consisting of this solution. Preferably, the measurements are carried out on-line automatically, for example controlled by an electronic and/or computer system. Even more preferably, said system is able to give signals or commands for correcting the acidity of the washing solution when necessary, thereby enabling the production of a fibre with a minimum amount of adhering acid or base.
The acidity of a washing solution for fibres can be determined by combining the results obtained by carrying out at least two analytical methods according to the invention. First, calibrations need to be carried out in order to determine the correlation of the results of each method with the acidity of the washing solution. Such a calibration is best performed by measuring on representative samples with different concentrations of salts or compounds, which are also -present in the washing solution for fibres of which the acidity needs to be determined, the different concentrations in the samples having been chosen according to an experimental design. An example of such an experimental design can be found in Table I of Example I. A calibration equation for each combination of analytical methods can be calculated from the results obtained by measuring in the samples, using a standard statistical technique, e.g. multiple linear regression or partial least squares, as described in H. Martens et al., Multivariate calibration, John Wiley and Sons, Chichester, 1989, or in D. L. Massart et al., Chemometrics: A Textbook, Elsevier, Amsterdam, 1987.
The process according to the invention can be used to remove adhering acid or base from any fibre. For example, the process can be applied to fibres produced using an acidic spinning dope solution comprising an aromatic polyamide, such as poly(p-phenylene terephthalamide), and sulfuric acid.
If a combination of analytical methods according to the invention comprising at least an ultrasonic sound measurement is used, then the propagation velocity of ultrasonic sound in a washing solution is measured using an ultrasonic device comprising an emitter and a receiver for ultrasonic sound. The device generates ultrasonic sound waves at the emitter, measures the time it takes before such a wave from the emitter passing through the solution arrives at the receiver, and calculates the propagation velocity as the distance between the emitter and the receiver divided by this time. Ultrasonic sound with a frequency of at least 50 kHz can be used for this measurement.
The propagation velocity (v) of ultrasonic sound in general depends on the density (d) and the adiabatic compressibility (c) of a solution according to the following equation:   v  =            1              c        *        d            
Consequently, if the density of the solution decreases, then the propagation velocity of ultrasonic sound in the solution increases. The density is dependent, int. al., on the temperature, the pressure, and the concentration of any compound or salt in the solution, such as sodium sulfate, sodium chloride, sodium hydroxide. Therefore, the propagation velocity of ultrasonic sound in a washing solution is not only dependent on the concentration of a compound or salt, but also on the temperature and the pressure in this solution. Preferably, the ultrasonic device used for measuring the propagation velocity of ultrasonic sound comprises a temperature sensor in order to enable a local adjustment of the temperature in the solution between the emitter and receiver.
The propagation velocity of ultrasonic sound in a washing solution is also slightly dependent on the flow of the solution in which the measurement takes place. Preferably, this propagation velocity is measured perpendicular to the flow direction.
If a combination of analytical methods according to the invention comprising at least a conductivity measurement, or a density measurement, or a refractive index measurement is used, then these measurements are carried out according to general conditions and instructions as described in F. McLennan and B. Kowalski., Process Analytical Chemistry, Glasgow, 1995.